A high-purity [001]-oriented monocrystal was deformed in tension and compression at small temperature increments between 100 and 4.2K. Two bends in the curve of flow stress versus temperature were observed which closely resembled the humps that had been found in body-centered cubic transition metals. The close correspondence of these non-uniformities in the refractory metals and in K suggested that they were intrinsic features of the motion of screw dislocations in a body-centered cubic lattice, and were not due to details of the interatomic potential. Above a temperature of 25K (lower bend), the flow stress was asymmetrical with respect to tension and compression. However, this asymmetry disappeared at 25K. This was consistent with a recent model which attributed the lower bend to a phase transition, of the screw dislocation core, from a low-temperature structure which glided on (110) planes, to a high-temperature configuration which could move only on (211) planes. Very general arguments showed that the motion of screw dislocations, via kink-pair formation on (110) planes, could not lead to a macroscopically observable asymmetry. Therefore, the asymmetrical behavior was expected to occur only in temperature and stress ranges in which the elementary glide step was on (211) planes.

W.Pichl, M.Krystian: Philosophical Magazine Letters, 1997, 75[2], 75-82